[NT-1809] Re-factoring add on selection + shipping rules logic

[singhhari]

📲 What

Because we make use of both shippingRules and shippingRulesExpanded there is some slightly confusing logic related to add on selection. In particular, we use shippingRules before add on selection and the expanded variant after. This led to a bug related to digital only add ons not rendering bc of a mishandled edge case. This PR simplifies the way we handle this flow and effectively uses shippingRule up until a particular add on is being mapped/filtered.

🤔 Why

Fixing a bug where a project with digital only add ons is returning no value for shipping rules and leading to a condition that looks like the UI timing out on a network request.

[codecov]

Codecov Report

Merging #1399 (42570c7) into master (d22ca87) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1399      +/-   ##
==========================================
- Coverage   86.00%   85.99%   -0.01%     
==========================================
  Files        1104     1104              
  Lines       97033    97025       -8     
==========================================
- Hits        83449    83441       -8     
  Misses      13584    13584              

Impacted Files	Coverage Δ
...ary/ViewModels/RewardAddOnSelectionViewModel.swift	98.49% <100.00%> (-0.03%)	⬇️
...iewModels/RewardAddOnSelectionViewModelTests.swift	99.38% <100.00%> (-0.01%)	⬇️

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[jgsamudio]

Great work! Thanks Hari!